1. Technical Field
Embodiments of the invention are generally related to optical circuits, and more particularly to a photodetector circuit with a tapered waveguide.
2. Background Art
The speed of a Germanium photodetector (PD) is typically affected by the resistance-capacitance (RC) characteristics of its contact area and the associated transit time for carriers generated by the PD to migrate from an intrinsic Ge zone to n-doped or p-doped contact areas. Although there is a general interest in scaling down a PD for improved speed, this interest is offset by the need to provide a large enough region for sufficient light absorption within the PD.
The dimension of a Ge PD along a direction of traveling light—such as a length of a waveguide Ge PD—is typically directly related to the area of the PD's p-i-n junction and to the region available for capturing light. If this dimension is too small, PD responsivity is impacted. As a result, this PD dimension is constrained by a minimum required value, which is usually wavelength dependent. Moreover, in the case of a Ge PD associated with a waveguide, PD size reduction is often constrained by an attendant need to reduce Si waveguide dimensions for coupling with the Ge PD input. Such scaling of the Si waveguide dimension makes coupling of light more challenging and could increase the loss of the waveguide.
For at least these reasons, photodetector scaling poses a challenge for improving the speed of optical communications. As successive generations of optics technology continue to scale in terms of data rates, there is an increasing desire for structures and fabrication techniques which provide for efficient and responsive photodetection.